In recent years the dry (carbon dioxide) reforming of methane (hereinafter “DRM”) has received increasing interest in both academia and industry. From the environmental point of view, the DRM uses CO2 and CH4 as raw materials, which are the main components of greenhouse gas and are believed to be related to the global warming, to produce CO and H2 (hereinafter “syngas”). Construction of DRM process units close to high CO2 production place, e.g., power plant, and utilization of flue gas as feedstock are considered to be an effective way to reduce CO2 emission. Additionally, syngas is an important intermediate for production of H2 gas and downstream chemicals such as methanol, dimethyl ether and liquid hydrocarbons as an alternative for petroleum-derived hydrocarbons. Each target required a certain H2/CO molar ratio that will vary according to the process used to produce syngas. The DRM itself or in combination with other reforming techniques such as steam reforming of methane (hereinafter “SRM”) and partial oxidation of methane (hereinafter “POM”) can produce syngas with tunable H2/CO molar ratio to meet different demands. Applications of DRM in the other fields such as solar energy transmission system and production of high purity CO (Calcor process) are also widely investigated.
The SRM is a conventional and mature process in industry which is used to produce hydrogen on a worldwide basis. In this process, heterogeneous nickel-based materials are the most commonly used catalysts. This kind of catalysts is also known to be active for the DRM. However, some issues such as sintering, coke formation and metal oxidation, especially coke formation, leading to deactivation of the catalysts, seriously hinder the application of the DRM in industry. Actually, the same issues exist in the SRM, but it can be overcome efficiently by increasing the H2O/CH4 molar ratio in the feedstock. Compared to the SRM, due to increased C/H molar ratio in the feedstock, the DRM causes more significant coking.
Many efforts have been devoted to reduce or inhibit the coking in the DRM. Addition into the catalyst systems of alkali or alkaline earth metals as promoters, which are believed to promote the chemisorption and dissociation of CO2 on the support, are widely investigated. The use of different support materials such as magnesia, ceria and zirconia, which are known to inhibit coking via different mechanisms, are also widely investigated. Bimetallic catalysts that uses synergistic effect of 2 different metals to create new chemical and physical properties, is another promising strategy to inhibit coking under DRM conditions. In that case, control of surface composition and overall catalyst structure are key parameters.
Bimetallic nanoparticles have attracted particular interests in the fields of catalysis and material science because of new properties such as improved activity, selectivity and stability, resulting from the interaction of two metals. For instance, in the case of naphta refroming process, supported ft-Pt catalysts are known to be more resistant to oxidative sintering than monometallic Ir catalysts, and supported Re—Pt catalysts are more tolerant to carbonaceous species than supported Pt.
Methods to prepare bimetallic nanoparticles can be divided into two types: physical (e.g., vacuum deposition, metal evaporation and sputtering) and chemical ones (e.g., co-impregnation and co-reduction). For both methods, there is a major challenge to precisely control particle size, size distribution, composition distribution and structure. Chemical methods, more attractive for very large-scale catalyst production, generally involve co- or successive impregnation methods for supported bimetallic nanoparticles and co- or successive reduction of two metal precursors in the presence of a stabilizer to prepare unsupported bimetallic nanoparticles in solution. However, due to the limit of preparation methods, a mixture involving monometallic nanoparticles of each metal and their alloys are typically obtained, and the structure of the final bimetallic nanoparticles, in particular surface composition and structure, is very difficult to control.
Thus, there is a need to produce catalyst systems in a controllable way so that the catalysts have a particular surface composition and structure.